The pulping of wood and other lignocellulosic materials with polysulfide liquors is known in the art. For example, U.S. Pat. No. 3,470,061, assigned to the same assignee as the present invention, describes a method for preparing polysulfide liquors by oxidation of the sulfide values with various manganese compounds. Previously referenced U.S. Pat. No. 3,551,286 describes the advantages obtained from pulping with a polysulfide liquor having a particular range of chemical values and claims the composition having that range of values. U.S. Pat. No. 3,653,842, also assigned to the same assignee as the present application, describes a particular process for oxidation and regeneration of the manganese compounds employed according to the proces of U.S. Pat. No. 3,470,061. In addition to these patents, there have been competitive patents describing pulping of wood and lignocellulosic materials with polysulfide liquors.
For example, Clayton et al., U.S. Pat. Nos. 3,664,918 and 3,664,919, describe particular processes for pulping with polysulfide liquors. The U.S. Pat. No. 3,664,918 specifically requires saturation of the wood chips with water before the polysulfide treatment, saturation being defined as from 180 to 200 percent water for soft woods and 150 percent water for hard woods. Following saturation, the chips are impregnated with an alkaline cooking liquor at a temperature below 110.degree. C. for about 60 minutes. The liquor is then removed and the impregnated wood chips are steamed at 170.degree. to 175.degree. C. According to the patent, if the chips are not impregnated with water before the treatment, then significantly lower yields are obtained.
In the Clayton et al. U.S. Pat. No. 3,664,919 impregnation is carried out at a low level, preferably below 6 percent polysulfide on wood. Again, the chips must be saturated with water prior to the treatment and the treatment is then carried out in essentially the same manner as in Clayton U.S. Pat. No. 3,664,918. Clayton U.S. Pat. 3,664,919 requires that the polysulfide be below 50 grams per liter and that the liquor-to-wood ratio, at the time of alkaline impregnation, be between 2.5:1 and 4:1.
Clayton U.S. Pat. No. 3,567,572 involves a multistep process which again requires an initial step of water saturation of chips before treatment. The saturated wood is then treated with polysulfide in the absence of an alkali, followed by treatment with alkali. A steaming is then carried out which may be in the presence of ammonia or hydrogen sulfide.
Other prior art patents dealing with polysulfide pulping of wood include U.S. Pat. No. 3,210,235--Ferrigan, Jr. et al; and U.S. Pat. No. 2,944,928--Kibrick et al. The Ferrigan, Jr. patent involves addition of elemental sulfur to white liquor in order to create a polysulfide pulping liquor, while Kibrick et al show heating in a pressure vessel for approximately 4 hours in order to accomplish digestion.